A filter-fan-unit designed as a modular unit in clean room applications is previously known, see for instance Babcock-BSH "Clean room-tunnel-module", prospect paper II/87. Adjacent to a side wall of this unit there is a fan with which the air is sucked in and transported into an air flow room, extended from the horizontal outlet of the fan to the opposite wall, then reversed 180.degree. and limited downwards by the filter means placed under the partition wall and at a distance therefrom. With this design the air will have long flow distances leading to correspondingly high flow losses and a correspondingly high energy consumption. This air guidance also makes it very difficult to obtain an even flow distribution over the filter surface. Further, this device has a comparatively high sound pressure level.
Efforts have been made to overcome the drawbacks of this known structure, e.g. according to EP,C,497296, wherein the flow room has been designed as one or several annular channels, and wherein at least one of the walls of the flow room is made of a sound-absorbing material. This is not an optimal solution either in view of flow stabilization and silencing of noise.